To address the potential role of HHV-8 in KS pathogenesis, we cloned over 80 kb of the HHV-8 genome (estimated size 160 kb). We used HHV-8 genomic clones as probes to screen complimentary DNA libraries prepared from HHV-8 containing malignancies. One of the isolated genes encodes a 28.9 kDa protein with homology to cellular cyclin D2 (mouse and human). Further analysis of the viral cyclin amino acid sequence revealed several features conserved amongst the D-type cyclin proteins and include: 1) a conserved region required for binding to cyclin-dependent kinases (cyclin box); 2) two putative retinoblastoma (Rb) binding sites; and 3) amino acid sequence important for protein stability. These presence of these structural features suggest that the viral cyclin protein when expressed, may have similar properties to the cellular cyclin D homologue. The transcriptional level of the viral cyclin was compared to those viral genes associated with lytic replication (major and minor capsid proteins) using reverse-transcriptase PCR. In circulating KS-like spindle cells and in early passage KS cells, only the viral cyclin was expressed. In situ hybridization of primary KS biopsies confirmed these results. Lymphoma cells chronically infected with HHV-8, cultured with inflammatory cytokines or the HIV Tat protein lead to an increase in viral cyclin expression without an increase in viral DNA synthesis. These data suggest that in KS, HHV-8 is latent. These results further implicate viral cyclin in changing the growth properties of HHV-8-infected endothelial cells.We hypothesize that aberrant expression of the viral cyclin is critical for KS development and/or disease progression. Furthermore, that inhibition of the viral and cellular cyclin in HHV-8-infected cells will result in regression of KS lesions. To test this hypothesis we propose the following specific aims: 1) To perform a phase I/II clinical study of flavopiridol in HIV- infected patients diagnosed with Kaposi's sarcoma; 2) To determine the effects of Flavopiridol on HIV and HHV-8 viral load; and 3) To study the mechanism of cell cycle inhibition by Flavopiridol in HHV-8 infected cells.